Our invention relates to a method of soldering two surfaces together with a view to the reduction of the thermal resistance of the soldered joint. The soldering method of our invention is particularly well suited for attaching electronic devices such as transistors, diodes, and integrated circuits to their mounts, although we do not wish our invention to be limited to this particular application.
Semiconductor appliances for power-handling applications are used widely in which the semiconductor devices are soldered to mounts made of, for example, sheet metal capable of high heat dissipation. Such sheet metal mounts formed in one piece with external leads for the semiconductor devices are referred to as lead mounts. A serial interconnection of such lead mounts, available in one form as a sheet metal punching, is collectively referred to as a lead frame.
A typical conventional method of soldering such semiconductor devices to sheet-metal lead mounts has been such that layers of a solder paste are formed, as by screen printing, on the respective mounts. Then the semiconductor devices are placed on the solder paste layers. Then the solder is heated and then allowed to solidify.
Heat dissipation is among the primary requirements of the soldered joints between such semiconductor devices and their mounts. The soldered joints must therefore be as low in thermal resistance as set forth hereafter. The thermal resistance of a soldered joint is known to decrease approximately in proportion with its thickness or with the amount of gas bubbles that are generated by the flux in use with the solder and entrapped in the joint.
As far as we know, only prior art approach to this problem has been to decrease the thickness of the soldered joint. To this end the semiconductor device has been rubbed against the mount via the solder layer, either manually or mechanically. Manual rubbing is of course objectionable by reasons of too much labor, time and expenses involved. We also object to mechanical rubbing because of the complex and expensive machinery required.